Out of consideration for environmental preservation, strict environmental standards for the composition of an exhaust gas discharged by combustion in the gas turbine engine are set up, and toxic substances, such as nitrogen oxide (hereinafter referred to as “NOx”), need to be reduced. In contrast, in the gas turbine engines for large-scale ground equipment and aircraft, a pressure ratio tends to be set high in order to reduce fuel consumption and increase an output, and this increases the temperature and pressure at an entrance of the combustion device. Since the temperature of the combustion easily increases by the increase in the temperature at the entrance of the combustion device, it is anticipated that NOx may rather increase.
Here, a combustion system adopting a lean premix combustion system which effectively reduces a NOx generation amount has been proposed in recent years. For example, a combined combustion system obtained by combining the lean premix combustion system and a diffusion combustion system has been proposed (see Japanese Laid-Open Patent Application Publication No. 8-28871 and Japanese Laid-Open Patent Application Publication No. 8-210641). In the lean premix combustion system, the air and the fuel are premixed and combusted as an air-fuel mixture whose fuel concentration is uniformized. Therefore, a combustion region where a flame temperature is locally high does not exist. In addition, the flame temperature can be wholly lowered by the dilution of the fuel. On this account, the NOx generation amount can be effectively reduced. In contrast, blow-off tends to occur at the time of low-load combustion. Moreover, since the diffusion combustion system combusts the fuel and the air while diffusing and mixing the fuel and the air, the blow-off is unlikely to occur even at the time of the low load, and a flame holding performance is excellent. In contrast, the diffusion combustion system has a problem with the reduction in the NOx generation amount. Therefore, in accordance with the combined combustion system, the reduction in the NOx generation amount can be achieved by the premix combustion at the time of high load while securing the combustion stability by the diffusion combustion at the time of start-up and low load.
For example, as shown in FIG. 6, the combustion device of the conventional combined combustion system adopts a swirl-type burner unit 85 configured such that a premix combustion burner (main burner) 84 including a radial swirler 83 having a fixed swirl vane is provided so as to surround an outer side of a diffusion combustion burner (pilot burner) 82 provided at a top portion 81a of a combustion liner 81 of a combustion device 80 and further configured to inject a pre-mixed gas P as a swirl flow into a combustion chamber.